This invention relates to an improved crystallization process for separation of a material that is crystallizable by freezing from other material. The process is performed at or near the triple point conditions of the materials. The process is characterized by a solid phase crystallizable material having a greater density than that of the liquid phase from which it is frozen, the solid phase tending to settle in the liquid phase by gravity. The solid phase is separated from the liquid phase to provide a drained bed in which the solid phase is melted.
The crystallization process is particularly useful in connection with the acid gas separation process disclosed in assignee's U.S. Pat. No. 4,270,937, dated June 2, 1981, the teachings of such patent being incorporated herein by reference. The patent discloses a comprehensive gas separation process which includes the use of a liquid carbon dioxide absorbent to separate certain acid gases and other impurities from a gas stream also containing lower boiling point components. There is also disclosed a triple point crystallization process for separating the carbon dioxide absorbent and absorbed impurities. In such crystallization process, the solid phase is formed and melted at spaced locations in the liquid phase. The solid phase is formed in a crystal forming zone in an upper region of the liquid phase and settles by gravity to a crystal melting zone in a lower region of the liquid phase. The solid phase is washed as it settles through the liquid phase intermediate the two zones. A condensing vapor is introduced into the melt zone for melting the submerged solid phase. The operating pressure in the melt zone is maintained above the triple point pressure by the static head of the liquid phase.
The prior melting technique within the liquid phase has not been entirely satisfactory, due to the inability to achieve sufficiently high melt rates in an energy-efficient manner. The major rate-limiting step in such prior technique is believed to be the resistance to latent heat transfer from condensing vapor to melting solid through the continuous liquid phase surrounding the solid crystals.
The location of the melt zone in a lower region of the liquid phase also tends to limit the transport of solid phase to gravity settling thereof through the liquid phase. This may comprise an undesirable process limitation in certain applications.
In a distinct separation task, the use of crystallization processes at triple point conditions for desalination of sea water is described in Rudd, Powers, & Sirola, Process Synthesis, pp. 259-280 (1973); Chemical Engineering, May 7, 1979, pp. 72-82; and Scientific American, December 1962, pp. 41-47. These prior art processes include direct contact heat exchange systems with and without the use of a secondary refrigerant comprising an in situ heat transfer component. In a single stage system, the use of evaporative cooling to form a bed of solid phase and the melting of the solid phase by direct contact with a compressed evaporation vapor are also described.
It is believed that these prior art desalination processes were not found to be entirely satisfactory due in part to their inability to provide economically high production rates. Specifically, the production rates were limited by the low vapor pressure of water at its triple point, which limits the pressure driving force to about 4.6 mm mercury. Further, the production rates were also limited by the contamination of the evaporation vapor with salt due to its entrainment during the extremely agitated conditions of evaporation (or, more descriptively, boiling) at high production rates. Further, the melt rate is believed to be a limiting factor due to the tendency of the melting to occur at the bed exterior surface only.